Recently, compact secondary batteries including Ni-metal hydride (Ni-MH) secondary batteries or lithium secondary batteries have been in increasing demand as portable instruments, such as mobile phones, notebook computers or camcorders, have been developed intensively. Particularly, since a lithium secondary battery using lithium and a non-aqueous electrolyte is the most powerful candidate capable of realizing a compact, low-weight and high-energy density battery, active research and development into lithium secondary batteries have been conducted. In general, a lithium secondary battery utilizes a transition metal oxide, such as LiCoO2, LiNiO2 or LiMn2O4, as a cathode active material, and lithium metal or carbon as an anode active material. Additionally, an organic solvent containing lithium ions is used as electrolyte between the two electrodes to provide a lithium secondary battery.
Since LiCoO2 was reported to be useful as a cathode active material for a lithium secondary battery in 1980, many studies about LiCoO2 have been made to date, and LiCoO2 have been widely used as a cathode active material for a common lithium secondary battery. However, because cobalt (Co) is one of the rarest elements on the earth, many attempts have been made to develop a novel cathode active material capable of substituting for LiCoO2. Particularly, LiFePO4 has a bulk density of 3.6 g/cm2, generates a high potential of 3.4V, and provides a high theoretical capacity of 170 mAh/g. Additionally, Fe is one of the most abundant elements on the earth, is inexpensive, and can be obtained at a low cost. Further, because LiFePO4 contains one electrochemically deintercalatable Li per Fe atom in its initial state, it is expected that LiFePO4 can sufficiently substitute for LiCoO2 as a novel cathode active material for a lithium secondary battery. In addition to LiFePO4, LiMPO4 containing a transition metal instead of Fe in LiFePO4 is known to one skilled in the art. It is an interesting thing that LiMPO4 provides a variable potential vs. Li depending on the kind of M.
A lithium metal phosphate represented by the formula of LixMPO4 (wherein M is at least one element selected from the group consisting of Fe, Mn, Co, Ni, Cu, Zn, Mg, Cr, V, Mo, Ti, Al, Nb, B and Ga, and 0.05≦x≦1.2) has an olivine-like crystal structure and undergoes little change in the crystal structure during charge/discharge cycles, and thus shows excellent cycle characteristics. Additionally, since the oxygen atoms in the crystal structure exist stably due to the covalent bonds with phosphorus atoms, a battery using a lithium metal phosphate is advantageous in that it shows little possibility of oxygen emission and has excellent safety even under high-temperature conditions.
For example, LiFePO4 powder may be obtained via a conventional solid phase reaction process or a sol-gel process. Such conventional methods frequently use expensive Fe2+ sources, such as iron acetate or iron oxalate, and require a high production cost. Meanwhile, when using Fe3+ sources, there is a problem in that reduction of Fe3+ cannot be accomplished sufficiently and Fe3+ compounds remain as impurities.
Therefore, there has been a continuous need for developing a novel method for preparing a lithium metal phosphate with high reproducibility via simple processing steps at a low cost in an industrial scale.